The present invention relates to making threaded joints and in particular to a process for proofing threaded joints to determine if they are satisfactory.
A threaded joint comprises work, an externally threaded fastener, and an internally threaded fastener. The externally threaded fastener operates with the internally fastener to apply an axial clamp-up load to the work, that is a compressive load to the work. Many threaded joints exist. One type has an externally threaded stud attached to the work with a threaded connection and which has a threaded section extending from the work for accepting a nut; an example of this type of joint is the lug bolts and nuts that attach a car wheel to its axle. A second type of joint uses a bolt that attaches directly to the work through a threaded connection; an example of this type of joint again is in the attachment of a wheel to its axle, but this time by lug bolts that thread into internal threads of the axle. A third type of joint uses a head of a bolt to bear on one side of the work and cooperates with a nut on the other side of the work to apply the load, with one or the other being permanently attached to the work, as by a weld. Another type of joint is the common and familiar one of a nut and a bolt that attach to the work only when used together; the nut and bolt being otherwise free. Nut and bolt joints can be press fit with a shank of a bolt in interference with the hole walls that it is in or with a clearance between the hole walls and shank.
All of these joints rely on an axial load applied to the work for their effectiveness. Without the desired clamp-up loads, the joints are failures. Sometimes threaded connections will appear to have desired axial loads, when in fact they do not. There are many reasons for this. Quite often, the torque applied to the fasteners in making a joint is used as a means to determine whether desired loads exist. Ideally, torque and clamp-up load correlate so that the amount of torque applied in making a joint indicates the amount of clamp-up load on the work. Unfortunately, the torque applied in making a joint is not always a fool proof sign of adequate clamp-up. Torque and clamp-up load correlate only when frictional resistance to turning between the fastener threads and between the work and the fasteners is within acceptable limits. Frictional resistance can increase beyond acceptable limits through different agencies. For example, damaged threads of the fasteners can increase the resistance to torque, indicating an achieved level of clamp-up much higher than the actual clamp-up. Increased frictional resistance that can result in a false indication of achieved load can also result from poorly performing prevailing torque lock nuts. (Nuts that use friction between their threads and the threads of cooperating fasteners to prevent joints from vibrating loose.) When the frictional resistance between the threads in prevailing torque lock, nut systems is too high, the torque level indicating desired clamp-up again does not correlate with the actual clamp-up, which is too low. Without exhausting all the causes of the failure of indicated torque to show actual clamp-up, other causes include a nut "shanking" on an unthreaded shank of a bolt and externally threaded fasteners bottoming in tapped holes. In either of the latter two cases, the indicated torque can be quite high while the actual clamp-up applied by the fasteners to the work can be zero.
Accordingly, there is a need for a method of determining whether threaded fasteners apply desired clamp-up load on work regardless of the setting torque applied to the fasteners.